The microstructures of an Al[sbnd]Zn[sbnd]Mg[sbnd]Cu alloy processed by high pressure torsion at room temperature and 200 °C were characterized by transmission Kikuchi diffraction and atom probe tomography. Hardening effects of different microstructural features including grain boundaries, dislocations and solute nanostructures were quantitatively calculated using existing models. Compared to the samples processed at room temperature, the samples deformed at 200 °C were of relative larger grain sizes, a lower dislocation density and more significant phase decomposition. Thus, the primary hardening effects, i.e. grain boundary hardening, dislocation hardening and cluster hardening subside at the elevated deformation temperature. Nevertheless, significant segregation of Mg and Cu formed at grain boundaries during deformation at 200 °C, which provides a remarkable hardening effect. The results revealed the importance of grain boundary chemistry on the mechanical strength of the ultrafine-grained materials.